WO1999016564A1

WO1999016564A1 - Mould pipe for a continuous casting mould for the continuous casting of steels, especially peritectic steels

Info

Publication number: WO1999016564A1
Application number: PCT/EP1998/005828
Authority: WO
Inventors: Adrian Stilli
Original assignee: Concast Standard Ag
Priority date: 1997-10-01
Filing date: 1998-09-14
Publication date: 1999-04-08
Also published as: ES2182361T3; DE59805207D1; JP2001518394A; AR015459A1; EP1019208A1; CZ289354B6; CZ20001187A3; ATE222150T1; EP1019208B1; ZA988870B; TW483783B; JP4393698B2; RU2203158C2

Abstract

The invention relates to a mould pipe for a continuous casting mould for the continuous casting of steels, especially peritectic steels. The inventive pipe has a first longitudinal section (1) incorporating a predetermined position for the level (h) of liquid metal in the mould, and a second longitudinal section (2) following on from said first section. The first longitudinal section (1) comprises a heat-insulating layer (16), the dimensions of which are such that the heat resistance of the mould pipe (10) in the first longitudinal section (1) is greater than that in the second longitudinal section (2). The heat-insulating layer (16) fills an area from the outer surface (11) of the mould pipe (10) to a distance equivalent to at most 75 % of the thickness of the mould pipe wall (dw) measured from said outer surface (11) of the mould pipe (10). By selecting the appropriate thickness profile for the heat-insulating layer in the direction of strand withdrawal (14), it is possible to set a given temperature profile on the inside of the mould pipe during a casting operation and optimise the growth of a strand shell.

Description

Mold tube for a continuous casting mold for the casting of steels, especially peritectic steels

The invention relates to a mold tube for a continuous casting mold for the continuous casting of steels, in particular peritectic steels, according to the preamble of claim 1 and a continuous casting mold with the mold tube.

The technology of continuous casting, in which, by cooling a molten metal on the walls of a mold cavity of a continuous casting mold, forms a continuous shell with a continuously increasing thickness and continuously extrudes a strand from an outlet opening of the continuous casting mold, is known to apply to peritectic steels, for example steels with a Carbon content of 0.1-0.14%, to problems that are particularly evident in the poor surface quality of the strands produced. Such quality defects are undesirable, especially since further processing of the strands often leads to unacceptable quality defects in the subsequent products.

As is known, one cause of the problems mentioned can be seen in a phase transition to which peritectic steels are subjected at a temperature just below their solidification temperature and which is associated with a considerable volume contraction. In the continuous casting of peritectic steels, this phase transition takes place during the initial solidification of a strand shell under conditions in which the strand shell which is still formed is of low mechanical stability and, as a result of the phase transition, forms an uneven surface which only lies selectively on the mold cavity wall, with which Result that solidified strands have a porous or cracked layer on the surface. As is known, in continuous casting of peritectic steels, an improved quality of the strand surfaces can be achieved by influencing the initial solidification of the continuous shell in a region of the continuous casting mold comprising the pouring level by reducing the heat dissipation from the steel melt or the continuous shell. This reduction in heat dissipation in the area of initial solidification is usually realized with the aid of continuous casting molds, which are equipped with a heat barrier on the steel-side surface of a longitudinal section of the mold cavity wall. The heat barrier is dimensioned and the longitudinal section dimensioned so that the heat flow density on the one hand

The area of the initial solidification is reduced, but on the other hand is large enough in the longitudinal sections adjoining the heat barrier to achieve sufficient growth of the strand shell over the entire running distance of the strand in the mold cavity.

Several concepts are known for providing the mold cavity walls of a continuous casting mold in the region of a partial length, which includes the mold level realized in the casting operation, with a heat barrier on the surface delimiting the mold cavity.

From JOS 1-224 142 a mold intended for the production of peritectic steels is known, the cavity wall of which consists of one

Pipe body with a cylindrical insert made of steel or other materials arranged on the pouring side and having a higher thermal resistance than the material forming the tubular body. This mold, the features of which form the preamble of claim 1, has the disadvantage that the use forming the heat barrier is susceptible to wear and special measures which make the mold more expensive are necessary to prevent cracks or deformations of the mold cavity wall due to the thermal loads during the casting operation counteract.

An alternative concept for the formation of a heat barrier is in JOS 1-170 550 using the example of one for the production of slabs from peritectic steel certain plate mold disclosed. The surfaces of the mold cavities on the side of the mold cavity made of copper have bores in a region encompassing the pouring mirror position, which are optionally filled with nickel, stainless steel or a suitable ceramic material. This alternative concept has the disadvantage that - apart from the susceptibility to wear of the fillings of the bores - it cannot be used on pipe molds for small strand formats, for example billet formats, for manufacturing reasons, since the inner sides of the mold pipes are only insufficiently accessible for suitable processing.

The invention has for its object to contribute to solving the problems mentioned and for this purpose a mold tube which is equipped with a heat barrier that can be produced using simplified manufacturing technology and is arranged at the mold level and has improved wear resistance, and a corresponding one provided with a mold tube To create continuous casting mold.

This object is achieved by a mold tube, which is characterized by the entirety of the features of claim 1, and a continuous casting mold with the features of claim 10.

The mold tube according to the invention has a first longitudinal section including a predetermined pouring mirror position and a second longitudinal section adjoining the first, the first longitudinal section comprising a heat-insulating layer which is dimensioned such that the thermal resistance of the mold tube in the first longitudinal section has a greater value than in the second longitudinal section. The mold is characterized in that the heat-insulating layer fills an area between the outer surface of the mold tube and a distance of at most 75% of the wall thickness of the mold tube, measured from the outer surface of the mold tube. The heat-insulating layer of the mold tube according to the invention is arranged on or near the outside of the mold tube and does not extend to the inner surface of the tube. The mold tube can therefore be produced from a tubular body that can be machined on the outside in order to provide it with the heat-insulating layer. Machining can be carried out using conventional methods, even for tubular bodies that are suitable for the manufacture of mold tubes with a small inner diameter and, due to their geometric dimensions, cannot be machined on the inside, or only at great expense.

In casting operation, the heat-insulating layer in the area of the first longitudinal section increases the temperature on the inside of the mold tube. Because the distance of the heat-insulating layer from the inner surface of the mold tube is at least 25% of the wall thickness of the mold tube, the wear of the mold tube during casting operation is due to the thermal and mechanical

Material stress in the area of the first longitudinal section is reduced compared to a mold tube which is equipped with a heat-insulating layer of the same thickness on the inside of the mold tube.

In the mold tube according to the invention it is possible to dimension the thickness profile of the heat-insulating layer

Temperature distribution, which occurs in the casting operation on the inner surface of the mold tube, has to be defined in order to influence the growth of a strand shell in the region of the first longitudinal section in a targeted manner. This degree of freedom is used in the mold according to the invention in order to optimize it with regard to the production of peritectic steel strands. In order to optimize the quality of cast products made of peritectic steel, the temperature on the inner surface of the mold tube in the area of the first longitudinal section should be as high as possible during the casting operation. As a result, the initial solidification of the molten steel sets in at a distance as far as possible from the pouring level, with the effect that the ferrostatic pressure of the melt increases with that Distance from the pouring level increases, counteracts a local detachment of the strand shell that is being stimulated by the peritectic phase transition from the inner surface of the mold tube and thus favors the formation of a smooth strand surface. On the other hand, the temperature on the inner surface of the mold tube cannot be arbitrarily high during the casting operation, since the material properties of the mold tube have a limiting effect. For example, a mold tube made of copper is known to have an unacceptably short service life after it has been heated to a temperature above a critical temperature of 450 ° C., the so-called softening temperature.

In an advantageous embodiment of the mold tube according to the invention, the thickness of the heat-insulating layer is therefore dimensioned such that the temperature on the inside of the mold tube in the casting operation does not exceed a predetermined critical temperature T _κ .

In a further embodiment of the mold tube according to the invention, the outer surface of the mold tube is of stepless design at the boundary between the longitudinal sections. This embodiment is particularly suitable for use in molds with water jacket cooling on the outside of the mold tube. Since the water jacket in such molds is usually only a few mm thick and its thickness along the mold tube must be precisely controlled, a stepless design of the transition between the two longitudinal sections enables a particularly simple construction of the water jacket cooling.

In a further development of the mold tube according to the invention, the heat-insulating layer is embedded in a tubular body made of metal or a metal alloy. Favorable thermal and mechanical properties of the mold tube are achieved if the tube body is made of copper or a copper alloy and the heat-insulating layer is made of a metal, for example nickel or chromium. These materials are good on each other in terms of their coefficient of expansion matched so that a nickel or. Chrome layer is characterized by good adhesion and high wear resistance.

Further embodiments of the mold tube according to the invention are thermally with regard to cooling the outer surface of the

Mold tube designed with a coolant in such a way that the temperature of the inner surface in the region of the first longitudinal section at most reaches a predetermined critical temperature and is approximately constant in at least one section of the first longitudinal section. In this way, the initial solidification of the strand shell can be delayed up to a particularly large distance from the pouring mirror and a particularly smooth strand surface can be achieved after passing through the peritectic phase transition. In order to achieve a temperature profile that is as constant as possible in the longitudinal direction, the thickness d of the heat-insulating layer must increase at least in a section between the pouring mirror position and the second longitudinal section in the direction of the second longitudinal section.

Various embodiments of the mold tube according to the invention are explained below using schematic figures.

Show it:

1A: an example of the mold tube according to the invention in side view;

1B: a cross section along the line I-1 in FIG. 1A;

Fig. 1C: a cross section along the line II-II in Fig. 1A;

2A: a longitudinal section along the line III-III in FIG. 1C for a specific thickness profile of the heat-insulating layer;

FIG. 2B: a longitudinal section as in FIG. 2A, but for a different thickness profile of the heat-insulating layer; 3: curves of the thickness d of a heat-insulating layer according to FIG. 2A as a function of the wall thickness d _{w of} the mold tube for a predetermined wall temperature;

4: a dimensioning of a heat-insulating layer as a function of the wall thickness d _{w of} the mold tube for a given profile of the wall temperature;

1A shows an example of the mold tube 10 according to the invention, shown in side view, with a mold cavity 20, a pouring opening 12 and a pull-out opening 13 for a strand (not shown). The direction of strand extraction provided in the casting operation is indicated by an arrow 14. The mold tube 10 has a first longitudinal section 1 and a second longitudinal section 2, the longitudinal section 1 comprising a casting level position h provided in the casting operation and the longitudinal section 2 adjoining the longitudinal section 1 in the strand pull-out direction 14. The mold tube 10 consists of a tube body 15 with a heat-insulating layer 16 in the region of the longitudinal section 1.

1 B and 1 C show cross sections of the mold tube 10: FIG. 1 B shows a cross section in the plane II marked in FIG. 1A in the region of the longitudinal section 1, FIG. 1 C shows a cross section in the one marked in FIG. 1A Level II-II in the area of the longitudinal section 1. As can be seen from FIG. 1 AC, the heat-insulating layer 16 is arranged on the outside 11 of the tubular body 15. The mold cavity 20 has, for example, a square cross section with rounded corners. This selection is arbitrary. The mold tube according to the invention can be equipped with any cross-sectional shapes customary in continuous casting practice. 2A and 2B show longitudinal sections along the line III-III in FIGS. 1B and 1C and identify two different embodiments of the mold tube 10 according to the invention, which differ in the design of the thickness profile of the heat-insulating layer 16 in the longitudinal direction of the mold tube . In both cases, the heat-insulating layer 16 is embedded in a depression on the outside of the tubular body 15. In these examples, the outer surface 11 of the mold tube 10 is stepless at the edges of the longitudinal section 1.

The tubular body suitably consists of copper or a copper alloy. Metals such as nickel or chromium, which can be applied to the tubular body 15 using conventional methods, for example plating or electrochemical processes, are expediently suitable as materials for the construction of the heat-insulating layer. However, other materials, for example ceramic materials, can also be used for the construction of the heat-insulating layer, provided that they have a lower thermal conductivity than the tubular body 15 and are suitable with regard to their adhesive properties and their wear resistance.

The embodiment of the mold tube 10 according to the invention shown in FIG. 2A is characterized in that the heat-insulating layer 16 has an essentially constant thickness in the region between the pouring mirror position h and its edge adjoining the longitudinal section 2, which is denoted by d in FIG. 2A. having. With this geometry, in the casting operation with uniform cooling of the outer surface 11 of the mold tube 10, the temperature on the inner surface of the mold tube 10 would be maximum from a point located at the mold level h

Decrease temperature in the strand pull-out direction 14, especially since a strand shell forming in the region of the longitudinal section on the inner surface 25 of the mold tube 10 has a increasing thickness in the strand pull-out direction 14 and ensures that the heat flow between the surfaces 25 and 11 of the mold tube 10 along the Strand pull-out direction 14 decreases. The temperature profile established on the inner surface 25 of the mold tube 10 can be modified in a targeted manner by a corresponding variation of the thickness of the heat-insulating layer 16 in the strand pull-out direction 14 in order to optimize the strand shell growth. The exemplary embodiment of the mold tube 10 according to the invention shown in FIG. 2B is characterized in that the heat-insulating layer 16 grows in a wedge shape from a thickness d to a thickness b in the region between the pouring mirror position and its edge adjoining the longitudinal section 2. The thicknesses d and b can be selected in relation to the wall thickness d _{w of} the mold tube 10 so that the temperature profile at the

The inside 25 of the mold tube 10 is approximately constant in the strand pull-out direction 14 and reaches a predetermined value. The detailed temperature profile is correlated with the strand shell growth on the surface 25.

The tubular body 15 is generally designed for use at a temperature below a maximum, critical temperature T _κ . The mold tube 10 can be designed for continuous casting of steel in terms of heat technology, provided that the outer surface is cooled by the application of coolant. So that the temperature on the inner surface 25 of the mold tube 10 does not exceed a predetermined critical temperature T _κ , the thickness d of the heat-insulating layer 16 at the casting level position h should be according to

dw λ _w [10 T _L + T _s - - 11T _K ] dd <-f. = d MAX (D

[1-f] α [T _s -Tκ] [1-f] be dimensioned with

λ _w : thermal conductivity of the mold tube 10 in the second longitudinal section

2; f: ratio λ _w / λi, where λi means the thermal conductivity of the heat-insulating layer 16; T _κ : critical temperature;

T _s : temperature of the steel on the inner surface 25 of the

Mold tube 10;

T _L : temperature of the coolant. α: heat transfer coefficient for the transition between the

Coolant and the heat insulating layer 16.

In Fig. 3, d = d _{MA is} shown graphically according to equation (1) as a function of the wall thickness d _w for the two parameters f = 4 and f = 10, where T _κ = 450 ° C, Ts = 1480 ° C and the following values representative of water cooling are assumed α = 30000 W / (m ² * K) and T _L = 40 ° C. T _κ = 450 ° C is a characteristic empirical value for copper. The two parameters f = 4 and f = 10 are, for example, representative of a mold tube 10 with a tube body 15 made of copper and a heat-insulating layer 16 made of nickel (f = 4) or steel (f = 10). As can be seen in FIG. 3, the ratio d _M Aχ dw of the mold tube 10 decreases with increasing wall thickness d _w . The smaller the thickness d of the mold tube 10, the greater the share of the thickness of the heat-insulating layer in the total thickness d _{w of} the mold tube 10 in order to reduce the temperature on the inside 25 of the mold tube 10 in the longitudinal section 1 to the critical temperature T. _κ , in the given example T _κ = 450 ° C, to raise. Furthermore, for a given wall thickness d _w , the d _MA χ / dw is the greater the smaller f, ie the greater the thermal conductivity λj of the heat-insulating layer. Experience has shown that the wall thickness d _{w of} the mold tube 10 should typically be approximately 10% of the side length of a cross section of the mold cavity 20. If the mold tube 10 is designed for small billets with a side length of the cross section of approximately 10 cm, the thickness ratio d _M Aχ d _w becomes approximately 75% for f = 4. For d Ax d _w ≥ 75% and f <4, the manufacture of the mold tube 10 from a solid tubular body 15 becomes problematic, especially since the mechanical stability of the tubular body 15 when realizing the depression on the outer side 11 of the heat absorbing layer 16 Tubular body 15 is unduly affected. Continues to increase increasing ratio d ^ xx. / d _w the effort in realizing the heat-insulating layer 16, in particular in production processes in which the heat-insulating layer 16 is built up by continuously applying thin layers of a suitable material. For the realization of the heat-insulating layer 16, the parameter range f> 4 is therefore preferred in addition to the condition d _M A d _w ≤ 75%.

4 is for the mold tube 10 in the event that the thickness of the heat-insulating layer 16 in the strand pull-out direction 14 increases from the thickness d at the position of the casting level to the thickness b according to a thickness profile which is determined such that in the casting operation along the thickness profile the temperature at the inner surface 25 is represented as the ratio b / d _w varies as a function of the wall thickness d _w constant. The relationships b / d _w and d / d _w can be determined from equation (1) and FIG. 4 in the event that the critical temperature T 1 is realized along the thickness profile in the casting operation. A comparison with FIG. 3 provides the corresponding values for the special case T _κ = 450 ° C. 4 is not dependent on f.

The length of the mold tube 10 is typically 80-100 cm. The length of the length section 1 is preferably in the range 10-15 cm, the pouring mirror position preferably being located in the upper quarter of the length section 1.

In the above-mentioned exemplary embodiments, the heat-insulating layer 16 is always embedded in a recess in the tubular body 15 in such a way that the outer surface 11 of the mold tube 10 is of stepless design. In the context of the inventive idea, it would also be possible to dispense with embedding the heat-insulating layer 16 in a depression or to provide the outer surface 11 with a continuous formation. The surfaces 11 and 25 of the mold tube according to the invention could also be provided with coatings made of suitable materials.

Claims

claims

1. Mold tube for a continuous casting mold for the continuous casting of steels, in particular peritectic steels, with a first longitudinal section (1) including a predetermined casting level position (h) and a second longitudinal section (2) adjoining the first, wherein the first longitudinal section (1) comprises a heat-insulating layer (16) which is dimensioned such that the heat resistance of the mold tube (10) has a greater value in the first longitudinal section (1) than in the second longitudinal section (2), characterized in that the heat-insulating layer

Layer (16) an area between the outer surface (11) of the mold tube (10) and a distance of at most 75% of the wall thickness (d _w ) of the mold tube (10), measured from the outer surface (11) of the mold tube (10) , fills out.

2. mold tube according to claim 1, characterized in that the outer surface (11) of the mold tube (10) at the boundary between the longitudinal sections (1, 2) is stepless.

3. mold tube according to one of claims 1 or 2, characterized in that the thickness (d) of the heat-insulating layer (16) is dimensioned such that the temperature on the inside (25) of the

Chill tube (10) does not exceed a predetermined critical temperature T «.

4. mold tube according to one of claims 1-3, characterized in that the heat-insulating layer (16) is embedded in a tubular body (15) made of metal or a metal alloy.

5. mold tube according to claim 4, characterized in that the mold tube (10) is thermally designed for continuous casting when cooling the outer surface (11) by applying coolant, wherein the thickness d of the heat-insulating layer (16) at the pouring mirror position (h) according to

JW λw [10 T _L + T _s - 11T _κ ] d <

[1-f] α [T _s -T _κ ] [1-f] is dimensioned, with d _w : wall thickness of the mold tube (10) in the first longitudinal section (1); λ _w : thermal conductivity of the mold tube (10) in the second longitudinal section (2); f: ratio λ _w / λi, where λi means the thermal conductivity of the heat-insulating layer (16); T _κ : critical temperature;

T _s : temperature of the steel on the inner surface (25) of the mold tube (10);

T: temperature of the coolant. α: heat transfer coefficient for the transition between the coolant and the heat-insulating layer (16).

6. mold tube according to claim 5, characterized in that f> 4.

7. mold tube according to one of claims 5 or 6, characterized in that the thickness (d, b) of the heat-insulating layer increases in a section between the pouring mirror position (h) and the second longitudinal section (2) in the direction of the second longitudinal section.

8. mold tube according to claim 7, characterized in that the increase in thickness of the heat-insulating layer (16) is dimensioned such that the temperature on the inside (25) of the mold tube (10) is approximately constant in the area of the section during the casting operation.

9. mold tube according to one of claims 4-8, characterized in that the heat-insulating layer (16) made of a metal, for example nickel or chromium, and the tubular body (15) made of copper or

Copper alloy is built.

10. Continuous casting mold for the continuous casting of steels, in particular peritectic steels, with a mold tube (10) according to one of claims 1-9.

11. Continuous casting mold according to claim 10, characterized in that a device for supplying the outer surface (11) of the mold tube (10) with a coolant, for example a water jacket cooling, is provided.